In recent years, electric powered vehicles such as hybrid vehicles and electric vehicles receive great attention as environmentally friendly cars. Such an electric powered vehicle includes a power storage device such as a secondary battery and a motor generator receiving electric power from the power storage device for generating a driving force. The motor generator generates a driving force at a time of starting or acceleration and also converts kinetic energy of the vehicle into electric energy for recovery into the power storage device at a time of braking.
As a motor generator mounted on such a motor vehicle, a permanent magnetic synchronous machine is often used, because of easiness of increasing magnetic flux density and power regeneration. Specifically, an interior permanent magnet synchronous machine is frequently employed in which driving torque (reluctance torque) generated by asymmetry of magnetic reluctance can be used in combination.
Permanent magnets are generally known to have magnetic coercive force changed according to environmental temperatures. For example, when a ferromagnetic material that is a main component of a permanent magnet is exposed in a high environmental temperature exceeding a Curie temperature marking a phase transition, the magnetic coercive force of the permanent magnet decreases, possibly causing irreversible demagnetization.
Japanese Patent Laying-Open No. 2001-157304 discloses a rotating electric machine for a hybrid car in which demagnetization of a magnet due to a temperature increase can be prevented. The hybrid car includes first and second rotating electric machines and a control device. The control device estimates a temperature of a permanent magnet of the first rotating electric machine based on data input for control of an engine and the first and second rotating electric machines. The control device estimates a temperature of an armature coil from the temperature of the permanent magnet to set the maximum carrying current value based on the armature coil temperature. The control device limits the current value in the armature to the maximum value or lower.
A method of estimating a temperature of the permanent magnet as disclosed in Japanese Patent Laying-Open No. 2001-157304 is, for example, as follows. First, a ratio K between the absolute values of a torque command value and output torque of the first rotating electric machine is calculated. Then, the magnet temperature is estimated by substituting the calculated ratio K into a map associating the magnet temperature with the ratio K.
In the foregoing temperature estimating method, the estimation accuracy may vary according to the difference in structure between the rotating electric machines. When the estimation accuracy is low, the estimated temperature may be lower than the actual magnet temperature. If the estimated temperature is lower than the actual temperature, it is likely that the rotating electric machine continues to operate without a temperature increase of the rotating electric machine being suppressed. Accordingly, the possibility that demagnetization of the permanent magnet occurs becomes high.